Suspension polymerization is commonly conducted in an aqueous suspending medium, by suspending discrete droplets of monomer in the aqueous medium, initiating a flee-radical polymerization, and continuing the polymerization until the suspended droplets have formed solid, spherical particles. Such particles, especially when formed by copolymerization of a monoethylenically unsaturated monomer and a polyethylenically unsaturated, crosslinking monomer, are particularly useful as intermediate materials in the production of ion exchange resins. Many common monomers, such as styrene, .alpha.-methylstyrene, methyl methacrylate, ethyl acrylate and the like, are not soluble in water, and thus are well suited to suspension polymerization. However, some monomers, as for example acrylic acid, methacrylic acid, hydroxyethyl methacrylate, acrylonitrile, acrylamide, methacrylamide, vinyl pyridine, dimethylaminoethyl methacrylate and the like, are soluble in water to a significant extent.
When suspension polymerization is attempted with a monomer which is partially or fully water soluble, monomer partitions into the aqueous phase. In the case of fully water-soluble monomers, the suspended droplets may never even form, and even where the droplets form, several undesired phenomena occur during polymerization, such as the occurrence of emulsion, "popcorn" or precipitation polymers in the aqueous phase, caused by dissolved monomer forming insoluble polymer which precipitates from solution, or the formation of particle agglomerates caused by the presence of soluble polymers in the aqueous phase. The agglomerates lead to a polymer product with poor hydraulic characteristics, and the presence of polymer in the aqueous phase also leads to fouling of process equipment.
Techniques known to those skilled in the art for suspension polymerization of water-soluble monomers include saturating the aqueous suspending medium with a salt, e.g. an inorganic salt such as sodium chloride or sodium sulfate, to reduce the solubility of the monomer in the aqueous medium. This helps reduce, but not eliminate, some of the undesired phenomena. Most suspension stabilizers are insoluble or unstable in a high-salt, aqueous phase, and thus do not adequately protect the monomer droplets, allowing them to agglomerate.